The market for hybrid electric vehicles and electric vehicles is expected to expand due to the recent growing interest in environmental protection and stricter regulations. The power supply system used for them tends to increase, and the size of electric wires used is increasing. The use of aluminum wires is advancing as a measure to reduce weight to improve fuel economy. The destruction of the oxide film on the aluminum surface is important for the connection between the aluminum wire and the terminal, and it is considered that ultrasonic bonding is effective particularly for the connection between the large－sized aluminum wire and the terminal. In this study, we investigated the correlation between contact resistance and welding strength, which are important for connecting wires and terminals. As a result, good contact resistance was obtained with a welding strength of about 1/20 of the maximum welding strength, and even if the weld strength was increased, the contact resistance was constant.